The subject of the present invention is the recovery of silver present in sulfuric solutions.
In many metallurgies silver occurs in solution in a sulfuric medium, particularly in plants for recovering zinc in an electrolyte phase.
This phase is frequently poorly recovered and many studies have been conducted to recover this silver. Thus, it is possible to mention French Pat. No. 2,313,452, which has certainly contributed some progress without this progress being decisive because the process is not applicable in every case.
In addition, numerous processes exist in which silver passes into a sulfuric medium without being capable of being recovered.
The subject of the present invention is the recovery of silver in a sulfuric medium with the aid of sphalerite.
This aim is attained by means of a process for recovering silver in sulfuric solution, in which the said silver solution is contacted with a quantity of zinc sulfide which is at least equal to the stoichiometric quantity and the surface area of which is at least equal to K Ag.sup.170 x V, K being a constant, Ag being the concentration of silver in the solution expressed in kilograms per cubic meter and V the volume of the solution in cubic meters, the surface area being expressed in square meters and the value of K being greater than or equal to approximately 10.
It is quite obvious that the higher the coefficient K chosen, the better will be the recovery of silver, but the lower will be the concentration of silver in the sphalerite.
The examples which will be set out below show that it is possible to obtain very high silver concentration and that it can be desirable to carry out a countercurrent precipitation.
The silver-bearing sphalerite thus formed can be recovered by any means known per se, for example, by filtration, flotation, density separation or physical elutriation.
This recovery method is of particular interest because it is possible to introduce the sphalerite at any leaching stage where the silver occurs in a soluble form, which makes it possible to cause an accumulation of silver in the particles of sphalerite, a sulfur-containing compound which is easy to recover despite the presence of lead sulfate and zinc ferrite which are frequently found in leaching residues.
It is of interest to note that the content of zinc and of sulfate has very little effect on the recovery and on the final concentration of silver.
When silver-complexing ions are present in the sulfate medium, it will be appropriate to make a correction of the silver content and to replace the value of Ag by the value of Ag.sup.+ which is really free, a value which is easily determined from the knowledge, or measurement, of the silver complexing constant or constants with the said complexing agents. The above relationship then becomes S=K(Ag.sup.f).sup.2/3 (Ag.sub.t)/(Ag.sub.f)V, where Ag.sub.f is the free silver concentration, Ag.sub.t is the total silver concentration; this relationship is equally valid in the case of a suspension of a poorly soluble silver salt where the free silver concentration is at least equal to 10.sup.-7, preferably at least equal to 10.sup.-5.
It can also be noted that it is advantageous to employ zinc sulfide of extremely fine particle size so that the specific surface of the zinc is very high and that in this way it is possible to obtain simultaneously very high degrees of recovery and a high concentration.
Any zinc sulfides may be employed, whether they be of chemical origin or mineral origin (sphalerite). In general, for economic reasons, preference would be given to very finely ground sphalerite, in general a sphalerite whose d.sub.80 is below 10 micrometers.
The optimum conditions which must be combined in the silver sulfate solutions are as follows:
(1) Temperature: from ambient to 100.degree. C., preferably from 50.degree. to 80.degree. C. when atmospheric pressure is employed; higher temperatures may be chosen when it is intended to work at higher pressures. PA1 (2) Pressure: atmospheric pressure. In fact, the value of the pressure has only a minor effect on the method described in the present application. PA1 (3) pH: the pH has no marked effect on the process. The only limits to observe are: in the case of the lower limit, the pH value under which the sphalerite is attacked by protons to produce hydrogen sulfide which is released; in the case of the upper limit, the pH value beyond which the silver concentration becomes lower than 10.sup.-6 M. Preferably, a pH of between 1 and 4 will be chosen. PA1 (4) Redox potential: the solution potential should be such that it does not attack the sphalerite by oxidizing the latter. The potential may be fixed by any suitable means which does not entail the precipitation of silver by cementation. Preferably, use is made of sphalerite which, in this case, does not need to be finely divided. It is also possible to employ the same sphalerite in the same state of division as for the process, which makes it possible to carry out the reduction of the solution at the same time as the silver precipitation. It is self-evident that the quantity of sphalerite required is that corresponding to the reduction of the elements capable of oxidizing it. PA1 (5) Duration: the reaction is fast and may be estimated at less than one-half hour. For safety a time equal to one hour or between one hour and two hours will therefore be chosen.
The following non-restrictive examples are intended to enable the experts to determine readily the operating conditions which should be employed in each particular case.